Modification of Roots

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In this article, we shall study the modification of roots for the purpose of food storage, respiration, support, etc. Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals. They are modified for support, storage of food and respiration. We shall study the modification of roots for tap root system and adventitious system.

Modification of Tap Roots:

Modifications of Tap Root for Storage of Food:

Taproots of carrot, turnip and adventitious roots of sweet potato, get swollen and store food. The secondary roots remain thin. Hypocotyl, i.e. the embryonic region between cotyledons and radicle may also join the taproot in storing food. The stem is reduced and disc-shaped in the beginning and bears radical leaves.  Depending upon their shapes they are further classified into four types.

Type	Characters	Examples
Conical	Base is broad and tapers gradually towards the apex.	Carrot (Daucus carota) (गाजर)
Fusiform	Swollen at middle and tapering at both the ends (spindle shaped). It gradually tapers towards the apex. Half of the fleshy part is derived from the hypocotyl	Radish (Raphanus sativus) (मूली),
Napiform	Spherical at the base and sharply tapering towards the apex (top shaped or globular). It suddenly tapers towards the apex. The most of the fleshy part is derived from the hypocotyl	Beet (Beta vulgaris) (चुकंदर) , Turnip (Brassica rapa) (शलजम)
Tuberous	Thick and fleshy without any definite shape.	4 O’ clock plant (Mirabilis jalapa)

Modifications of Tap Root for Better Respiration:

The plants growing in saline, swamps, marshy places, and salt lakes are called halophytes. Such plants e.g.  Rhizophora growing in swampy areas (mangroves), many roots come out of the ground (negatively geotropic) and grow vertically upwards. Such roots, called pneumatophores. They help to get oxygen for respiration. The roots appear like conical spikes coming out of the water.  They occur in large numbers near the tree trunk. Exposed root tips possess Rhizophora minute pores (lenticels or pneumatothodes) through which roots respire. e.g. Rhizophora, Avicennia, Sonnerita, Heritiera (सुंद्री found in Sunderbans, Bengal).

Characteristics of Respiratory Roots:

• These are modified tap roots.
• These are non-green and non-photosynthetic
• They are found in marshy habitats like swamp and mangroves near seashores
• They grow vertically upward in response to gravity i.e. they are negatively geotropic.
• These roots are covered with cork and the gaseous exchange takes place through pores called lenticels

Nodulated Roots:

In legumes (pea family), the secondary roots of primary tap root bear small tubercles or swellings which are called root nodules. These nodules shelter nitrogen-fixing bacteria Rhizobium leguminosarum. They help in fixing atmospheric nitrogen into nitrates which can be absorbed by the roots.

Modifications of Adventitious Roots:

Modifications of Adventitious Root for Storage of Food:

Simple Tuberous Roots: 

These roots are creeping and become swollen and do not assume any shape. They are always borne singly. These roots arise from nodes of prostrate stem and enter in the soil. e.g. sweet potato (Ipomoea batatus) (शकरकंद).

Fasciculated Tuberous Roots: 

It is a cluster of adventitious roots for the storage of food. These roots have a definite shape. e.g. Dahlia, Asparagus (शतावरी)

Nodulose Roots:

Only apices of roots become swollen like single beads. e.g. mango, ginger, turmeric

Beaded or Moniliform Roots:

Roots alternately swollen and constricted which has beaded or moniliform appearance. e.g. Grasses, sedges, momordica.

Annulated Roots:

Looks like number of disc placed one above the other. e.g. Ipecac

Modifications of Adventitious Roots for Support:

Prop Roots:

The hanging structures that support a banyan tree are called prop roots. Roots develop from tree branches hang downwards and ultimately penetrate the ground, thus provide support to heavy branches. A banyan growing in Indian Botanical garden, Owrah (Kolkata) has nearly 1700 such prop roots and has a very large spread. The tree is about 200 years old. Another example is the mangrove plant Rhizophora.

Characteristics of Prop Roots:

• They arise from the branches of the stem.
• They hang down vertically and enter the soil.
• They are quite long.
• They behave like a pillar and give support to the plant.

Stilt Roots:

The stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem. These are called stilt roots. They are mainly found in monocots, shrubs and small trees. They grow obliquely downwards and penetrate the soil. Their primary function is to provide support to the plant. In plants like maize, bajra, sugarcane, jowar, they grow in whorls.  In screwpine(केवडा) or Pandanus (a tropical palm-like tree) these roots arise only from the lower surface of the obliquely growing stem to provide support. Another example is bamboo.

Characteristics of Stilt Roots:

• They develop from basal nodes of the stem.
• They grow obliquely from the stem.
• They are shorter in length
• They provide support to the plant as the ropes provide to the tent.

Climbing Roots:

Such plants produce roots from their nodes, by which they attach themselves to some support and climb over it. Weak climbers twine around and clasp the support with the help of climbing roots arising from their nodes. e.g. Money plant, black pepper (kali mirch), betel (pan). In Ivy, adhesive disc grows from climbing roots.

Clinging Roots: 

Special clinging roots arise, enter the crevices of support and fix the epiphyte. e.g. epiphytes orchids

Buttress Roots:

They are present at the basal part of the stem and spread in different directions in the soil. They are vertically elongated and horizontally compressed. They look like planks e.g. Ficus, Bombax, Terminalia.

Modifications of Adventitious Roots for Special Functions:

Epiphytic Roots:

Some plants like orchids grow on horizontal branches of big trees in the forest to get sunlight. They are autotrophic. These plants are called epiphytes. They develop special areal hanging roots called epiphytic roots. These roots are spongy. Due to the presence of velamen tissue are hygroscopic and have a porous wall. They absorb moisture from the atmosphere. e.g. vanda, dendrobium, etc. These roots are also called assimilatory roots due to their partial capacity of photosynthesis.

Sucking Roots or Haustoria or Parasitic Roots:

These are highly specialized and microscopic roots, developed by parasites to absorb nourishment from the host. In partial parasites penetrate only xylem element of the host and absorb water and minerals. E.g. Viscus album. In total parasites, they establish contact with both xylem and phloem of the host. Thus absorb water, minerals, and nutrients. e.g. Cuscuta, Orobanche, Viscum, Lorathus.

Floating Roots: 

Spongy, floating roots filled with air, arise from nodes of some aquatic plants, and help in floating and respiration. eg. Jussiaea

Photosynthetic or Assimilatory Roots: 

Roots which when exposed to sun develop chlorophyll, turn green and manufacture food. e.g. Tinospora (gilo) and orchids. In Tinospora, the roots arise as green hanging threads. Other examples are Taeniophyllum, Trapa ( Singhara), and Podostemon.

Characteristics of Assimilatory Roots:

• These are modified adventitious roots.
• These are green and photosynthetic
• They are found in diverse habitats like aquatic, terrestrial and epiphytes.
• They grow horizontally in response to gravity and hence referred as diageotropic similarly, they show branching pattern and hence they are also referred as plagiotropic.
• Gaseous exchange takes place through the general surface.

Mycorrhizal Roots: 

This is a symbiotic association between higher plants and fungus. In some plants, roots become associated with fungal hyphae. This association of a fungus with higher plants is called mycorrhiza. The fungus absorbs water and minerals from the soil, in turn, the plant provides organic food to the fungus. Example: Pinus, Monotropa

Reproductive Roots:

Adventitious roots of some plants develop buds that give rise to leafy shoots. These roots help propagation. Example: sweet potato

Contractile Roots:

These roots can be found on underground rhizome, bulb, tuber, corm, etc. of some plants. They maintain a proper level of the plant in the soil. Example: Cracus, Freesia, Canna.

Root Thorns:

In some plants, roots are modified by hard pointed thorn-like structures called root thorns. e.g. Pothos armatus and Acanthorrhiza.

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